Abstract: One analog signal is selected out of a plurality of analog signals by a multiplexer and a capacitor is electrically charged though such a switch as adapted to be closed/opened by a start signal, and thereafter the capacitor is caused to discharge with a constant current with the switch opened. A level discriminating signal is led out when a voltage of the capacitor discharging goes below a predetermined discriminating level after the switch is opened. A stop signal is generated as a result of a logical operation of the analog signal and the level discriminating signal by an AND gate. A processing circuit accepts the stop signal and calculates a digital value corresponding to a time period from the time that the switch is opened to the time that the level discriminating signal is generated, and then moves to a subsequent processing operation.

Abstract: Closed-loop air-fuel ratio control performed by detecting the concentration of a predetermined component in the exhaust gas, by calculating an air-fuel ratio correction factor depending upon the detected concentration, and by correcting the fuel amount in accordance with the air-fuel ratio correction factor even when the engine is warming up. This closed-loop air-fuel ratio control operation is stopped by fixing the air-fuel ratio correction factor to a predetermined value when the engine is under acceleration during warm-up.